1. Field of the Invention
This invention relates to an electrical logging system. In particular, this invention relates to an electrical logging system for determining the electrical resistivity of a subsurface formation through a cased borehole.
2. Description of Related Art
Conventionally such a system comprises voltage and current supply means, voltage and current control means, electrode support means, and voltage and current processing means for processing currents and voltages thereby determining said electrical resistivity.
It is well known to apply the above system to carry out measurements in a logging procedure in order to determine electrical resistivity of the formation through the casing of a borehole. Generally metal casings are applied, in particular those made of steel. Borehole depths may be up to several thousands of meters. For example such a formation may have a structure of oil bearing layers having a resistivity quite different from the surrounding layers. More in particular the case of subsequent water- and oil-bearing layers occurs. Depending on the hydrocarbon saturation such layers typically have resistivities in the range of 1 to 1000 .OMEGA.m.
Especially in situations of already existing boreholes having been cased, for example to update subsurface formation characteristics or to monitor production operating progress, it is of great interest to have reliable data.
As shown in U.S. Pat. No. 2,729,784, in the past attempts have been made to measure currents leaking from cased drill holes into the subsurface formation. Two axially spaced current electrode pairs housed in one and the same logging device were connected to the casing resulting in casing currents between the electrodes of each pair when energized. The electrical circuitry was chosen such that the two neighbor electrodes of said two pairs had the same electrical potential thus defining a casing part therebetween functioning as a quasi separate electrode from which currents leaked into the formation. By providing three additional electrodes in the casing part between said two pairs of electrodes both a symmetrical electrical potential distribution is set and voltage measuring is accomplished allowing the determination of the electrical resistivity of the subsurface formation.
However, it is well known that electrical measurements on casings are far from ideal because of roughnesses of the casing, for example caused by differences in casing thickness and corrosion spots. Thus casing resistances are affected substantially. In the above logging method such disadvantageous effects are combined with electrical measuring and control on small leakage currents starting from the above-mentioned short casing part between the electrode pairs. In view of the above, this means that only very small voltages can be measured whereas the reliability of such measurements has to be considered very low. Consequently large deviations in the subsurface formation resistivity should be expected.
From recent literature it is known to handle the above-mentioned situations and cases in a different way. For example U.S. Pat. No. 5,043,668 discloses a method and apparatus for determining the above said resistivity through the casing of a borehole. In particular, three current electrodes and three voltage electrodes are employed, five electrodes being electrically connected to the casing at different heights, measured in axial direction of the borehole, and one current electrode being arranged at a substantial distance therefrom within the earth's surface.
By means of this set-up both calibration measurements and resistivity determinations are carried out.
In the calibration measurement mode, a casing current between the two current electrodes connected to the casing induces two voltage indications between the three voltage electrodes which are connected to the casing and are arranged between said two current electrodes. Said indications reflect the casing resistances between said voltage electrodes. Again casing resistances are affected as explained above. Therefore said two voltages are supplied to electronic circuitry in order to control and to balance the respective proportional voltage indications.
In the resistivity determination mode the same voltage electrodes are used. However, now an electrical current is induced into the casing and into the formation from the upper current electrode which forms an electrical circuit with the current electrode arranged within the earth's surface. Clearly in this mode leakage currents, leaking into the formation, occur. Consequently in this mode, the voltage indications as detected by the above mentioned three voltage electrodes are more or less modified. Typically in this mode mainly casing currents, which are many orders larger than the leakage currents because of the corresponding difference between casing resistivity and formation resistivity, are inducing said proportional voltages. Notwithstanding the above, the differences between the voltages obtained from the above explained two modes of operation, although being very small, reflect said leakage currents. As a result thereof formation resistivity can be determined.
Also for this set up, it will be clear to those skilled in the art that only slight variations as to said differences, for example caused by corrosion spots, electrical perturbation, mechanical damage, or circuitry tuning, will result in substantial casing current errors. Consequently obtaining reliable leakage current determinations is very difficult.